Rotating mirror scanner



R. J. HEREGLD ROTATING MIRROR SCANNER Filed May ll, 1966 .Patented ug.25, i369 3,463,832 RTATENG MIRROR SCANNER Robert J. Hcrbold, Sunnyvale,Calif., assigner, by inesne assignments, to Technical perations,incorporated, Burlington, Erlass., a corporation of Delaware Filed May11, 1966, Ser. No. 549,225 int. (Il. H0411 3/02 ELS. Cl. 17d-f7.6 l()Claims ABSTRACT @F THE DIISCLSURE This disclosure -depicts an opticalflying spot scanning system including beam swinging means and a novelanamorphic lens in the form of a lar disposed contiguous to the scanplane in the path of the spot-forming beam for reducing spot jittertransverse to the beam path.

This invention relates to rotating mirror scanners of the kind whichproduce a television type raster and particularly to improvements forovercoming spot jitter which results from mirror imperfections causingangular deviations of the beam in a flying spot laser scanner.

One example of a typical use of the present invention is shown in myassignees copending application of Willard E. Buck and Randy J. Shermanfor High Speed Facsimile Method and Apparatus, Ser. No. 443,529. Theapparatus shown in said application utilizes a multi-faced mirror forsweeping a focused beam of light across a film to produce a rasterpattern. This will presently be explained more fully but here it willsufrice to understand that separate mirror faces cause a succession ofscans and as the film is moved during scanning, the individual scansacross the lm and progress in closely spaced relationship lengthwise ofthe film, if the mirror faces are not perfectly formed and parallel tothe axis about which they rotate, deviations in the scan path will occurresulting in uneven spacing of even overlap of the paths.

It is the object of this invention to provide means to overcome theabove described scan ray jitter and 'to provide means to overcome thisphenomenon without the very costly and tedious task of grinding andpolishing the mirror faces to the high degree of perfection required.

Further and more specific objects and advantages of the invention aremade apparent in the following specification wherein the invention isdescribed in detail by reference to the accompanying drawing.

ln the drawing;

FIG. 1 is a perspective schematic View of the principal optical elementsin a flying spot laser scanner;

FIG. 2 is a greatly enlarged fragmentary sectional view taken through afilar lens and its support used in the apparatus disclosed in FIG. l;

FIG. 3 is a modification of the clement illustrated in FIG. 2 employingtwo filar lenses to prevent a parallax effect which results from the useof a binocular optical system; and

FIG. 4 is a schematic view illustrating possible variations in positionof the lar in an optical system.

The scanner shown in FIG. 1 is basically the same as that disclosed inthe above mentioned copending application and it includes a light sourceYin the form of a laser schematically illustrated at 10. The beam oflight from the Vsource may be modulated by a device illustrated at 11which may be a Kerr cell or other similar device capable of receivingsingals representing varying intensity of light and capable ofmodulating the beam from the light source to correspond with the signal.The purpose of this, as is fully explained in the above mentionedapplication. is to enable the scanner of FIG. 1 to reproduce a record onfilm which is being scanned by another device -at a remote location. Themodulated beam is passed through a lens system comprising lenses l2 and13 to convert the very narrow laser beam momentarily to a considerablywider beam of collimated light which is intercepted by a beam splitter14 herein disclosed as the type having a half mirrored surface disposedat an angle to the beam so that one-half of the light is permitted topass through the surface while the other half is reflected towardoneside where it is intercepted by a mirror 15. The` twobeams thusproduced are shown at 16 and 17 and are directed toward lenses 20 and21, respectively, which focus them to very small points through a mirrorsystem to be described and substantially at the surface of a strip offilm represented at 23. This-strip of film is fed as from one reel toanother and is driven to move constantly in the direction of the arrowshown at 25.

At a point between rthe two reels and where the focused beams of lightscan the surface of the film, the film is confined by means (not shown)to a curved configuration, the curve being defined by the point of focusof the light beam as it is reflected from the surfaces of either of apair of rotating triangular mirrors shown at 27 and 28, These mirrorsrotate at high speed on a common axis and are usually driven either by aturbine or by an electric motor (not shown).

The beam of light 16 after being focused by the lens 20 passes (as shownin FIG. l) to a mirror 30 which directs it toward the rotating mirror27. One face of this mirror causes the focused point of the beam tosweep `across the surface of the strip of film 23 to produce a singlescan of a raster. The sweep of this beam is and since the mirrors 27 and28 are formed as equilateral triangles, the instant that one mirrorstops writing through this 120 sweep, the other mirror will startbecause, as seen in the drawing, the other part 17 of the split beam isfocused by a lens 2l to a mirror 34 which directs it to the rotatingmirror 28 to be swept across the surface of the film strip. The speed oftravel of the film strip is such that the line exposed or written on thefilm by the two mirrors acting alternately is immediately adjacent theline before it with a consequence that modulation of the intensity ofthe beam in accordance with a record on a remotely positioned film willproduce a facsimile or duplicate of the record on the film in themachine described.

It has been .found that the manufacture of rotating mirrors suitable foruse in the above-described apparatus is extremely diflicult because anydeviation from a plane which is parallel to the axis of rotation willcause the mirror to direct the focused spot away from the intended scanline in a direction longitudinally of the hlm. This results in jitter orrandom spacing which, if excessive, is intolerable. Investigation withone apparatus using triangular mirrors that were sufliciently perfectfor other optical uses showed random spacing as high as 50 microns witha focused scanning spot having a diameter of 5 microns. Costly andtime-consuming balancing and polishing of the mirrors used has reducedsuch random spacing to 15 to 2O microns, but far greater accuracy hasbeen achieved by the practice of the present invention which utilizes ananamorphic lens in the form of a filar lens between the collecting orfocusing lenses and the film surface. Such a lens is illustrated at 40in FIG. l, and also shown greatly enlarged in FIG. 2. yIt has beenlearned that glass and quartz fibers used optically for variouspurposes, say from 1 to 500 microns in diameter, make excellentcylindrical lenses which will focus exterior to themselves whenilluminated perpendicular to their axes with a cone of light. As shownin FIG. 2, the filar 40 is mounted between the edges of two pairs ofthin metal strips ill and 42 supported in any suitable housing member 43and spaced by shims 44 to which they are preferably adhesively secured.This entire assembly with the lar follows the curvature of the iilrn,and is very slightly spaced therefrom so that the focal point or focalline of the lilar occurs at the surface of the film. In FIG. 2 thelfocused beam from one of the lenses 2t) or 2l is illustrated, afterhaving been intercepted and swung by one 'of the rotating mirrors, at16A. This beam converges upon the lilar lil) which has a very shortfocal length and is converged thereby to a spot at the film surface. Thedifference of the image motion in the image produced by the fllarrelative to that produced by the lens has proven to be greater than l()to 1 in the apparatus described and, consequently, the random spacingproduced as a result 'of slightly imperfect mirror faces has beenreduced to less than live microns or one spot diameter.

The assembly shown in FIG. 2 may prove unsatisfactory lfor some purposesdue to the occurrence of parallax which results from the binocular orsplit beam system illustrated in FIG. l. For example, when light comesto lthe filar from two directions as it will from the two mirlrors 27and 28 in FIG. l, it will be focused on the iilm `of image plane in twodifferent positions spaced apart ilongitudinally of the film. This doesnot present a disiadvantage if the record being scanned or reproduced onthe lm of the present device uses a similar optical system, because inproducing a record with a raster it makes llittle difference which scansof the raster occur first or 5in what order they occur. lt is possible,however, to use the system of FIG. 2 without producing parallax byemlploying, as shown in FIG. 3, two lilars Si) and S1 in place of thesingle lar 4) of FIG. 2. In this case the filars are supported andspaced from the film in much the isame manner as shown in FIG. 2, buteach intercepts converging beams from the mirrors 27 and 2S representedhere at 27A Vand 28A, and these beams are focused at the same point orline on the image plane.

Referring again to FIGS. 2 and 3, the cones of light 16A, 27 and 28 areschematic representations and are intended to denote the portion only ofcones from a focusing lens which penetrates and is focused by the lar,ln practice, the iilar is disposed at a position within the cone wherethe diameter of the cone exceeds that of the filar. For example, in FIG.4, a lens 50 focuses light to a point 52 and a lilar 54 is disposedwithin the focused beam in advance of that point and in a position wherethe conical beam is larger than the filar. This permits deviation of thebeam without reducing the quantity of light entering and focused by thelar. The position of the lar should be such that it remains within thebeam when deviation occurs. Consequently, the intensity of the spotfocused by the iilar at the plane 56 will not be varied as a result ofdeviation.

It is also possible and in some cases desirable to locate the iilar inthe diverging rather than the converging part of a focused beam. This isillustrated in FIG. 4 wherein ,a second filar 5S is shown in analternative position beyond the focal point S2 to produce a focusedpoint at a 'plane 60. In either position of the lar illustrated in FIG.4 it receives less than the total light in the focused beam but limitedbeam deviation does not vary the intensity of light passing through theiilar.

Since the tilar is cylindrical is produces an astigmatic effect, and theimage produced at the film plane is a line rather than a dot or disc.This is undesirable since obviously it reduces the fidelity of therecord which is made. This undesirable effect can be corrected byastigmatizing the cones imaged by the spherical lenses 2G and 21. Thepresent invention avoids the high cost of custom made astigmatic lensesto correct this phenomenon by mounting the lenses 2G and 21 oil axis orat slight angle from 4l the ordinary position in which the central planeof the lens is normal to the axis of the beam. This angularityintroduces astigmatism which counteracts the astigmatic eliect ofthecylindrical filar.

I claim:

l. In a rotating mirror scanner in which a focused beam is swept acrossan image plane by a moving mirror surface to produce a raster, means toreduce the magnitude of random spacing between individual scanscomprising a anamorphic lens of small cross-section disposed continuousto the image plane in a position to receive the beam and focus it at theimage plane.

2. The invention of claim 1 in which separate rotating mirror surfacesare used.

3. The invention of claim .1 in which the anamorphic lens extends acrossthe image plane in the direction of the sweep and is parallel to theimage plane.

4. The invention of claim 1 in which the anamorphic lens is positionedto receive less than all of the light in the focused beam.

5. The invention of claim 4 with means for supporting said anamorphiclens and for masking the light at both edges thereof, said meanscomprising two pairs of plates disposed parallel to said plane to bescanned, one pair on each side of said anamorphic lens, the plates ineach of said pairs of plates being spaced a distance less than thediameter of said anamorphic lens, said anamorphic lens being capturedand supported between said pairs of plates.

6. The invention of claim l in which the beam to be focused is split andhas its two parts focused and directed toward the image plane atconverging angles, and a separate anamorphic lens intercepting eachfocused part of the beam in a position to refocus both at the same pointat the image plane to prevent parallax.

7. The invention of claim l in which a lens is used to focus the beamand in which the lens is mounted at an angle to its axis whichcompensates the astigmatic effect of `the anamorphic lens.

S. The invention of claim 1 in which the filar is disposed in theconverging portion of the beam so remote from the focal point thereofthat the beam is of greater diameter than the iilar.

9. The invention of claim 8 in lwhich the filar is disposed inthe'diverging portion of the beam.

lt). A flying spot optical scanning system comprising:

means for deecting an input light beam across a surface to be scanned;and

anamorphic lens means located in the locus of said beam as it traversessaid surface to be scanned with its axis lesser power being parallel tosaid locus, said anamorphic lens means being disposed contiguous to saidsurface to be scanned, said lens means serving to reduce jitter in saidbeam in a direction transverse to said locus.

References Cited UNITED STATES PATENTS 1,870,465 8/1932 Rathbun.

2,045,921 6/1936 Peck 178'7.6 2,059,221 11/1936 Fessenden 178-7.62,464,793 3/1949 Cooke.

2,560,614 7/1951 Walker 178-7.6

ROBERT L. GRIFFIN, Primary Examiner B. LEIBOWITZ, Assistant Examiner US.Cl. X.R.

gg@ UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3463882 Dated August 26, 1969 Invent0r(s) Robert J. Herbold It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 4, claim l, line 1l, "continuous" should read --contiguous-; and

Column 4, claim 8, line 43, "filer" should read --anamorphic lens.

ollJnLU Huu SEALED JAN 6 1970 (SEAL) Attest:

Edward M-Fmhek WILLIAM E. somma JR Amming Officer Commissioner' ofPatents

